Homocysteine is an amino acid, derived from methionine, may be converted to cysteine

Its metabolic pathways require vitamins B12, B6 and folate

Elevated levels may be hereditary (due to mutations in these pathways) or acquired (due to deficiencies of vitamins B12, B6 or folate, renal failure, carcinoma, hypothyroidism or medications)

Elevations in homocysteine are associated with increased risk of arterial and venous thrombosis and atherosclerosis, based on retrospective case control studies

Prospective studies show a weak positive association with arterial thrombosis, and no definite association for venous thrombosis

Homozygosity or heterozygosity for the C677T mutation in MTHFR gene (methylene tetrahydrofolate reductase), which is involved in homocysteine metabolic pathway, does not appear to be a risk factor for thrombosis, but may be significant in folate - deficient patients

Although MTHFR was previously thought to be associated with thrombosis, newer data suggests this test is not useful in the first - line evaluation of thrombosis

Mutation in the methioninesynthetase gene (MTR) also can lead to increased homocystein levels but, as with MTHFR mutations, are not thought to be associated with thrombosis

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